1. Field:
This invention pertains to the treatment of combustion gas to remove sulfur gases. It is particularly directed to the reduction of sulfur emissions from stack gases, and an improved approach to providing finely divided carbonaceous fuel in association with finely divided reactant.
2. State of the Art:
The in situ control of SO.sub.2 stack emissions by the injection of reactants (notably calcium-based sorbents) has been actively investigated for many years. Various approaches to flue gas desulfurization are described, for example, in U.S. Pat. Nos. 2,718,453; 3,781,408; 4,273,750; 4,350,670; 4,366,134; and 4,424,197. Each of these patents, the disclosures of which are incorporated herein by reference, discloses a process whereby a gas containing SO.sub.2 is reacted with a reactant to produce a sulfur-containing reaction product which is retained in the cinders, filters or scrubbers of a combustion system.
Recent trends in utility flue gas desulfurization are discussed in an article (incorporated herein by reference) by J. Z. Abrams of Bechtel Group, Inc., entitled "The Status of Lime in Stack Gas Scrubbing" (for presentation at the annual meeting of the National Lime Association, May 3-4, 1982). This article describes the 73 coal-fired power-generating units currently equipped with operational FGD (flue gas desulfurization) systems," about 111/2% of the coal-fired electrical generating components of the United States of America in 1982. The vast majority of such systems (including all of the commercially operating systems), involved the contact of flue gas with a calcium-based slurry to produce calcium sulfite (CaSO.sub.3) and in some cases, a smaller amount of calcium sulfate (CaSO.sub.4).
The article, "The Application of Dry Additives on Reducing SO.sub.2 Emission for Brown Coal Fired Boilers" by Klaus R. G. Hein and W. Glaser, Rheinisch-Westfalisches Elektrizitatswerks, 4300 Essen, Federal Republic of Germany, discusses a phenomenon known as "natural retention" of sulfuric oxides in the fly ash in combustion systems utilizing solid fossil fuels with high concentrations of basic components (basic metal oxides or their equivalent). The article (incorporated herein by reference) speculates that a catalytic surface reaction MO (M=basic metal)+SO.sub.2 +1/2O.sub.2 .fwdarw.MSO.sub.4, occurs during combustion so that a portion of the SO.sub.2 gas produced is retained as solid sulfates in the combustion residue. The authors suggest that the same reaction can be used for further SO.sub.2 removal if a basic oxide is added. A high specific surface area and/or a high porosity of the metal oxide reactant is recommended. It is postulated that natural retention of in excess of 50% of the SO.sub.2 produced may be achieved. It is further suggested that the Ca/S value in the fuel may be increased by the addition of calcium-based materials to the fuel, in some cases prior to grinding. The authors concluded that sulfur retention does not relate directly to SO.sub.2 emission and that there is no direct functional correlation between sulfur retention and the Ca/S ratio in the fuel. Nevertheless, they believed that certain Ca/S values needed to be maintained if a maximum SO.sub.2 -emission was not to be exceeded. Such values were believed to be practical in brown coal but were determined to be inoperative in the case of bitumnous coals because of the phenomenon of "dead burning" of additives.
A paper entitled "Limestone Injection With an Internally Staged Low NO.sub.x Burner" was presented by Joel Vatsky and Edmund S. Schindler of Foster Wheeler Energy Corporation, at the EPA/EPRI First Joint Symposium On Dry SO.sub.2 and Simultaneous SO.sub.2 /NOx Control Technologies in San Diego, Calif., November 1984. This paper (incorporated herein by reference) describes efforts which have been made to decrease sulfur emissions from power plants by injecting pulverized limestone and lixe into the combustion zone of a boiler. It also makes reference to previous efforts to mix limestone with coal, and injecting the mixture into a test furnace. Purportedly, 50% SO.sub.2 emission reduction had been obtained by others prior to the work done by Foster Wheeler. The specific work reported on achieved between 40-60% SO.sub.2 emission reduction at Ca/S ratios of 2 to 4, utilizing pressurized injection of hydrated lime in a special low NO.sub.x burner. The same level of SO.sub.2 emission reduction had been obtained by others with "super fine" sorbent particles (98%-325 mesh) injected through or near the tertiary air ports of the burner. The coal and limestone were pulverized in a Foster Wheeler MBF-16 mill. The tests demonstrated that when the coal and limestone were copulverized, unacceptable levels of SO.sub.2 capture were obtained. The postulated explanation for this unsatisfactory sorbent utilization was deadburning of the calcined limestone. To avoid deadburning, Foster Wheeler developed a novel in-burner sorbent control method. This method effected "acceptable" levels of sorbent utilization; i.e., SO.sub.2 emission reductions of 50% with limestone sizings equal to that of the coal (91.5%-200 mesh, 100%-50 mesh).
The state of the art prior to this invention has thus been that the retention of SO.sub.2 in the ash produced by a burner could be increased by adding basic metal oxide reactants to the solid carbonaceous fuel delivered to a burner, thereby reducing SO.sub.2 emission by as much as 60%. The industry has recognized, however, that copulverization of the fuel and the adsorbent (reactant) should be avoided because of the inherent phenomenon of deadburning. Moreover, superfine sizing of the adsorbent wasn't deemed to be necessary or desirable, although high-surface adsorbent was recognized as being more reactive than lower surface area forms of the same material.
There is a growing trend in certain countries, notably the U.S.A., to rely upon coal and petroleum coke as fuels. Large users have made significant progress in controlling sulfur emissions, but through the use of expedients is not practical for smaller users. U.S. Pat. No. 4,531,461 discloses a microfine powdered coal combustion system which is suitable for delivering pulverized coal to the burner systexs typical of smaller users. The system comprises an enclosed coal metering and grinding section which delivers microground coal directly to a burner. The specific surface areas of the xicrofine powder produced by the grinding section (typically -325 mesh) is substantially higher than previously available solid fuel burner fuels. There results an elongated flame similar to an oil flame, characterized by rapid combustion, quick turn-down capability and complete combustion. The ash content of the solid fuel provides an excellent radiation source so that the coal flame and its products of combustion provide better heat transfer than an oil or gas flame of equal heat release rates. The sulfur dioxide emission in the flue gases from this system is less than would be expected based upon the sulfur content of the fuel. The sulfur which is not emitted is retained by the ash, which is removed by various means from the flue gases. Despite the significance of reduction of sulfur emission experienced with this system, unacceptable levels of SO.sub.2 emission still occur.
SO.sub.2 emission from power plants and other fossil fuel burners remains a significant environmental problem. A need for improved in situ SO.sub.2 capture or retention procedures remains.